A) Field of the Invention
This invention relates to a data compression technique for a digital image signal, and more in detail, relates to wavelet transformation.
B) Description of the Related Art
An image, especially a multi-level image, includes an enormous amount of data, and it causes a problem that the data amount is enormous when an input signal is transmitted or stored. Therefore, a highly efficient coding for compressing the data amount will be executed by omitting lengthiness of the image or by simplifying the image to a degree that deterioration will not be recognized before storing or transmitting the image signal.
Conventionally, in the JPEG Standard, the image data is divided into blocks of an 8×8 pixel, and the image data is transformed into DCT coefficients by executing discrete cosine transform (DCT) for each block, and thereafter data compression will be executed. When a compression rate in quantization is increased, a data compression rate becomes large. Small data after the quantization is abandoned so that the transformation is a lossy (non-reversible) transformation. Since the data compression is executed by each block in the DCT, so-called block distortion may be appeared at the borders of the blocks.
In the JPEG 2000, a wavelet transformation is suggested as a transformation process to be executed before the quantization. The wavelet transformation is not executed by each block, but the input data is sequentially processed. Therefore, deterioration of compound image becomes difficult to be visually recognized.
FIG. 6A is a block diagram schematically showing a wavelet transformation apparatus. Input signal X is an image signal of a raster-scanning format. A series of the image data is supplied to a low-pass filter (LPF) 61 and a high-pass filter (HPF) 62, and each of them outputs a low frequency component Y of the wavelet transformation coefficient or a high frequency component Y−1.
FIG. 6B is a plan view schematically showing the wavelet-converted image signal. A low frequency component L and a high frequency component H of the wavelet transformation coefficients are respectively arranged into horizontally divided regions 65 and 66 and form a screen 64 of the wavelet transformation coefficients.
The wavelet transformation is executed not only in a horizontal direction but also in a vertical direction. By the wavelet transformation in the vertical direction, the low frequency component L of the horizontal direction for the region 65 is divided into a low frequency component LL of the horizontal and the vertical directions for the region 65-1 and a component LH of which horizontal direction is high frequency and vertical direction is low frequency for region 65-2. Similarly, the horizontal direction high frequency component H for the region 66 is divided into a component HL of which horizontal direction is high frequency and vertical direction is low frequency for region 66-1 and a component HH of which horizontal and vertical directions are high frequency for region 66-2.
As described in the above, frequency division can be executed for each component obtained by the above process. By repeating the wavelet transformation, the frequency components in the image signal can be divided into a desired degree.
As a wavelet transformation, 9×7 format and 5×3 format are well known. In the wavelet transformation, operations for extracting a high frequency component and a low frequency component to a series of the image signals are repeated. For example, Japanese Laid-Open Patent 2001-285643 suggests storing a wavelet coefficient necessary in later operation in a storing device for simplifying the structure of an operation apparatus. Also, for example, Japanese Laid Open Patent 2002-359849 suggests using a turn-around rotation unit for executing the wavelet transformation in vertical direction and horizontal direction in the same operation circuit.
As the wavelet transformation, although, the 9×7 format and the 5×3 format are used, each filter used in each format has different structures. Therefore, when the wavelet transformation apparatus that can use both of the 9×7 format and the 5×3 format is consisted, the number of the components increases, and the structure becomes complicated.